net/virtio: fix in-order Tx path for packed ring

net/virtio: fix in-order Tx path for packed ring

When IN_ORDER feature is negotiated, device may just write out a
single used descriptor for a batch of buffers:

"""
Some devices always use descriptors in the same order in which they
have been made available. These devices can offer the VIRTIO_F_IN_ORDER
feature. If negotiated, this knowledge allows devices to notify the
use of a batch of buffers to the driver by only writing out a single
used descriptor with the Buffer ID corresponding to the last descriptor
in the batch.

The device then skips forward in the ring according to the size of the
batch. The driver needs to look up the used Buffer ID and calculate the
batch size to be able to advance to where the next used descriptor will
be written by the device.
"""

But the Tx path of packed ring can't handle this. With this patch,
when IN_ORDER is negotiated, driver will manage the IDs linearly,
look up the used buffer ID and advance to the next used descriptor
that will be written by the device.

Fixes: 892dc798fa9c ("net/virtio: implement Tx path for packed queues")
Cc: stable@dpdk.org
Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>